This study compared the growth status of different batches of introduced Loblolly pine (Pinus taeda) and conducted a comprehensive evaluation of the growth of 40 newly introduced Loblolly pine families to identify those with excellent early growth.

The study subjects were 40 loblolly pine families introduced and afforested in Yingde and Lechang, Guangdong. Traits such as tree height, diameter at breast height (DBH), and survival rate at 3 and 5 years were investigated. The results were compared with three test forests introduced in 1997 to assess the effectiveness of the current introduction. Variance analysis of tree height, DBH, and individual tree volume at two locations was conducted using R and ASReml software, and breeding values were calculated to select families with superior early growth.

1) Significant variations were observed among families of the introduced Loblolly pine (Pinus taeda) for all measured traits. In the Yingde trial forest, the mean tree height was 5.95 m, mean DBH was 14.00 cm, and mean individual volume was 0.044 9 m³. In the Lechang trial forest, these values were 6.09 m, 9.94 cm, and 0.024 2 m³, respectively—all exceeding those of the 1997 trial forest; 2) Based on breeding values for volume, four superior families were identified for early growth at the Yingde site: AGCLB-37, AGCLB-38, AGCLB-29, and AGM-025, with mean values of 6.15 m, 15.53 cm, and 0.0569 m³, outperforming the control group CK2. At the Lechang site, four families—AGM-225, AGM-005, AG-674, and AG-373—showed superior early growth, with means of 6.38 m 10.47 cm, and 0.027 6 m³, surpassing CK2.

The trial forests in Yingde and Lechang exhibited robust growth, with the introduced families demonstrating superior performance compared to those introduced in 1997. Four elite families were selected from each site (Yingde and Lechang), achieving volume-based genetic gains of 26.73% and 14.05%, respectively. Subsequent efforts should focus on selecting superior individual trees from these families. The chosen individuals can be used to enhance the second-generation breeding population and establish a third-generation breeding population, thereby enriching the genetic resources for Loblolly pine (Pinus taeda) improvement.

To solve the issues of abundant flowering but low seed-setting rates, which has severely impacted the production of improved seeds and the supply of seedlings from the orchard. research on the synchronization of flowering periods (SFP) among clones in the S. superba seed orchard was conducted to provide reference value for optimizing the clone configuration and achieving abundant seed and fruit production in the orchard.

In 2023, a preliminary study was conducted on 10 representative S. superba clones with high flowering intensity and abundant fruiting. To enhance sample diversity and representativeness, 53 additional clones were included in 2024, resulting in a total of 63 clones. The flowering phenology and flowering synchronization of all tested clones were surveyed and analyzed.

1) The total flowering duration (TFD) for the clones of S. superba ranges from 9 to 19 days, with variations in flowering phenology among different clones; 2) The results of cluster analysis and ANOVA (Analysis of variance) indicated that based on the timing of the first flowering period (FFP), the clones can be divided into three categories. There are extremely significant differences (P<0.01) in flowering periods among different clones: The FFP of the early-flowering clone was on May 3rd, with a TFD of 14.63 days. The FFP of the mid-flowering clone was on May 14th, lasting for a TFD of 14.02 days. The FFP of the late-flowering clone was on May 21st, with a TFD of 13.00 days. 3) Within the seed orchard, approximately 36.9% of the clone combinations have a flowering synchronization index (C) greater than 0.6, while 29.44% of the clone combinations have a C value less than 0.1. Within the annual period, the coefficients of variation for the C value among clones ranged from 61.05% to 240.72% in 2023 and from 37.87% to 164.52% in 2024, respectively. Between years, there are some differences in the C value, but it remains relatively stable overall. The average C value is higher for combinations of clones of the same type compared to combinations of different types, and the average C value for mid-flowering clones is higher than that for early- and late-flowering clones.

Currently, C value of flowering among clones within the seed orchard is relatively low. Therefore, attention should be given to the differences in flowering phenology and flowering synchronization among S. superba clones. Select mid-flowering clones and combinations of similar clones, screen out compatible clones with different provenances and a high C value to plant adjacently, and introduce measures such as artificial pollination to facilitate pollination, thereby enhancing the seed and fruit production and quality in the seed orchard.

To evaluate the growth of the second-generation core population open-pollinated progeny of loblolly pine, screen for superior families and individual trees, analyze the paternal relatedness of superior individual trees using chloroplast SNPs, and conduct a secondary selection of these superior trees to identify candidates for the third-generation core population.

Using the second-generation core population open-pollinated progeny of loblolly pine in Guangdong Province as research objects, a survey of growth traits at the age of 6 years was conducted. Genetic evaluation was performed using SAS and R software. A combined selection approach was used to identify 35 superior individual trees for SNP marker development and relatedness analysis.

1) Significant differences in growth traits were observed among families, with performance exceeding that of the controls. The average height, diameter at breast height (DBH), and volume of the progeny forest were 6.56 m, 14.67 cm, and 0.055 7 m³, respectively, which were higher than those of CK1 (6.26 m, 13.00 cm, 0.042 7 m³) and CK2 (6.31 m, 13.17 cm, 0.042 6 m³); 2) Based on volume breeding value ranking, 25 superior families were selected with average height, DBH, and volume of 6.65 m, 15.34 cm, and 0.061 0 m³, respectively. Using a combined selection method, the top two individuals with the best overall performance were selected from the top 10 families, and the best individual was selected from families ranked 11th to 26th. Family Ⅲ-68-4 was excluded due to the lack of superior individuals. A total of 35 superior individual trees were selected; 3) Chloroplast genome sequencing of the superior individual trees revealed their paternal relatedness. To control relatedness, the number of individuals potentially originating from the same paternal parent was limited to two. After adjusting the number of superior individuals while maintaining the number of superior families, 31 superior individual trees were finally selected.

The 35 superior individual trees had an average height of 7.27 m, DBH of 19.63 cm, and volume of 0.101 5 m³, with an average volume genetic gain of 17.51%. Considering their relatedness, the 31 selected superior individual trees are recommended for inclusion in the third-generation core population of loblolly pine.